1960s BGW HBO 500W

The HBO 500W is a general purpose double-ended compact arc source that was introduced in 1938 by Osram in Germany. The only electrode seal that was available when this lamp was developed was C. Bol's graded glass on wire construction (Philips, the Netherlands). The HBO was thus essentially an hermetically-sealed spherical quartz bulb attached to two metal tubes. Following the invention of the moly foil seal by D. Gabor in the late 1930s (Siemens Brothers, England), the mercury arc lamp was redesigned during the 1940s into the form shown here. An additional electrode was added in order to facilitate ignition using a high-voltage generator outside the main ballasting circuit. Electrodes were still kept symmetrical so as to enable a lamp operation on both DC and AC circuits. In the latter case the current-limiting ballast took the form either of a resistor or of an inductor.

In order to ensure a stable operation, this lamp is run in the vertical position so the arc axis is aligned with the convection flow inside the burner. In Osram's original HBO 500 design,the arc was located in the center of the quartz vessel and a chimney was placed around the top electrode so as to channel the convection flow and collect the evaporated tungsten material. This complex design was improved by removing the cup and by shifting the electrodes to a lower position. This way, the burner has a more homogeneous temperature profile and a smoother convection flow pattern.

Because of its ease of use and versatility, the HBO 500W became a widely used light source for optical (projection), technical, and scientific applications. Osram's design was then adopted by many lamp manufacturers, like BGW of East Germany (followed by Narva in 1969), who produced this particular model for several decades. The popularity of this lamp began to decline during the 1970s as superior DC-driven mercury arc sources were introduced in increasingly larger types. During that period an HBO 500W version without ignition electrode was released for a used on modern power supplies with built-in ignitor. The original version of the lamp was eventually phased out in the late 1980s.